It has been common practice in the manufacture of drive shaft assemblies, also known as propeller shafts, to produce the assembly by welding a length of cold-rolled, non-alloyed low carbon steel tubing to an end fitting such as the yoke of a universal joint or a coupling. The welding of the end fitting to the tubing, however, results in a heat affected zone due to high temperatures in the vicinity of the weld. These high temperatures produce regions of varying strengths and hardness in the heat affected zone. The base metal in the weld area is subjected to all temperatures from above the melting point down to room temperature. The metallurgical changes in this heat affected zone are many and are determined by the temperatures reached and the subsequent cooling rates.
The microstructure of the weld nugget is columnar. The material is molten during the time current flows, and during solidification the grains first form at the solid boundary. The grains can only grow in one direction resulting in the typical columnar grain structure in this region. Another distinct zone immediately adjacent the solid boundary also reaches a high temperature but not enough to melt the material. At this elevated temperature considerable grain coarsening develops.
Proceeding away from the weld toward the unchanged base metal in another zone that had been heated above the upper critical temperature but not hot enough to give coarsening. In that zone the grain is refined, or in other words, annealed. The hardness is greatly reduced because of the annealing treatment and the tensile strength is lowered to about 65,000 psi. From that zone outward there is a gradual change in metallurgical phase and tensile strength back to the original unchanged base metal.
Because of the substantial weakness in the heat affected zones of drive shafts manufactured in this way, it has been common design practice to simply provide a very large section of cold-rolled steel tubing, heavy enough to withstand the torsional forces to be expected in service, even in the weakened, weld-annealed areas. The shafts are thus designed for their weakest points, the heat affected annealed areas, and are considerably heavier and stronger than necessary throughout the remainder of their lengths. Such shafts are substantially heavier in weight, thicker in gauge, and sometimes larger in diameter than drive shafts produced according to the present invention described below.
A related problem in drive shafts has been torsional vibration. This problem generally increases with an increase in shaft diameter. Therefore, when the diameter of the shaft is reduced, according to the present invention, the torsional vibration problem is generally reduced.
Other problems known in the art are inherent system unbalances associated with attachment runout and large polar-moments of inertia, which increase inertial excitation accompanying shaft non-uniform rotation.
Heat treating of certain structures subsequent to welding operations is known. One of the purposes of the heat treatment is to relieve heat induced stresses resulting from the welding operation. However, the known art does not disclose the manufacture of a drive shaft assembly from a hardenable alloy steel tube, by first performing the welding operations and then heat treating the assembly within the specific temperature limitations and according to the accompanying steps of the method of the present invention described below.
The drive shaft assembly of the invention is produced by first welding a hardenable steel alloy tube to the yoke of a universal joint or other end fitting. The entire assembly is then heat treated and tempered according to specific procedures which include heating and quenching to form martensite and, subsequently, tempering. The result is a drive shaft assembly of superior physical properties. The tubing section of the shaft is of uniform hardness and uniform residual stress, with no adversely affected zone in the vicinity of the weld. A uniform martensitic steel structure is achieved. Drive shafts produced according to the invention, as compared with conventionally manufactured prior art shafts of the same size and weight of steel tubing, have been found to exhibit a two- to three-fold increase in torsional strength.
A drive shaft assembly according to the present invention is considerably stronger, for a given size tubular shaft, than drive shafts produced according to prior methods of manufacture described above. Thus, a shaft assembly of reduced size and weight can be produced according to the invention, with at least equal torsional strength to that of a prior art conventional shaft assembly.
By reducing the diameter of the drive shafts, according to the present invention, the spring rate is changed and the torsional vibration problem is reduced. A light weight, high strength drive shaft constructed according to the present invention, permits a reduced torsional spring rate which is sometimes required to de-tune a system torsional vibration response.
Such improved drive shafts provide a reduced shaft polar moment of inertia, which in turn reduces inertial excitation accompanying shaft non-uniform rotation, which is a consequence of universal joint non-uniform motion.
Therefore, light weight, high strength drive shafts, according to the present invention, provide multiple advantages. Inherent system unbalance associated with attachment runout is reduced. Not only is the basic weight of the unit lowered while maintaining the structural strength requirements, but the reduction of static weight reduces deflection between supports and therefore reduces vibrational problems. In some applications, a center drive shaft bearing may be eliminated without resulting in undesirable vibration. System lateral response frequency, a function of shaft support characteristics and shaft weight, is increased when using shafts according to the present invention.